This invention relates to an improved compression refrigerant heat pump and, in particular, to alleviating problems associated with overcharging the outdoor coil of the heat pump with refrigerant when the heat pump is operating in the heating mode.
A standard refrigerant air conditioning unit will contain an indoor coil for absorbing heat from a comfort zone into the refrigerant and an outdoor coil for rejecting the heat in the refrigerant to the surrounding ambient. In addition to handling the heat absorbed into the refrigerant during cooling, the outdoor coil must also handle the heat of compression developed by the unit compressor, and the heat generated by both the compressor motor and one or both of the coil fan motors. As a result, the outdoor coil of a standard air conditioning unit for providing cooling is considerably larger than the indoor coil simply because the outdoor coil must perform more work than the indoor coil.
As is well known, the standard air conditioning system can be employed to provide heat to a comfort zone by thermodynamically reversing the cooling cycle to draw heat from the surrounding ambient and rejecting the absorbed heat into the comfort zone. To this end, a reversing valve is connected to the unit compressor to reverse the flow of refrigerant through the coils. When in a heating mode, the larger outdoor coil acts as an evaporator and the indoor coil as a condenser in the system.
As can be seen, the functions of the outdoor and indoor coils are reversed when the system is in a heating mode. The condenser, which is now the indoor coil, is smaller than the evaporator or outdoor coil. Accordingly, the small condenser is not able to store as much liquid refrigerant as the outdoor coil, which acts as the condenser, than when the system is in the cooling mode. Because only a small amount of liquid refrigerant can be held back in the indoor coil, and thus an over-abundance of refrigerant is found in the outdoor coil producing an unwanted condition known as overcharging. This condition can be alleviated to some extent by use of regulated expansion devices such as thermal expansion valves for controlling the quality of return gas. These devices are relatively expensive and require the use of complex controls. Non regulated expansion devices such as capillary tubes and the like are less expensive and complex, however, they are incapable of regulating the return flow to the compressor when the heat pump is in a heating mode, and as a consequence, liquid or wet refrigerant may be delivered directly into the compressor pumping cavity thereby adversely effecting the operation of the system and compressor lifespan.
Industry standards require that refrigerant entering the compressor be superheated to about 10.degree. above saturation in order to protect the compressor components. Most systems employ an accumulator tank in the suction line of the compressor to prevent liquid refrigerant from entering the compressor. Although use of an accumulator insures that only superheated vapors enter the compressor when the system is up and running, cold refrigerant nevertheless can be drawn into the compressor outlet during start up.
Along with the problem of low superheat, heat pumps employing rotary compressors encounter low oil temperatures whereupon the compressor discharge gas is able to condense into the oil. This, in turn, results in bearing problems and can lead to compressor failure.
Since the cooling mode determines the heat pump system design, the heating cycle will always be overcharged with refrigerant unless special precautions are taken. As noted, non-regulated expansion devices cannot close down the refrigerant flow between coils to eliminate overcharging. In an ideally adjusted heat pump system, therefore, more refrigerant is required in the cooling mode than in the heating mode. Attempts to equal the charges at some average value lowers the cooling performance to an unacceptable level and does not totally solve the problems associated with overcharging when operating in the cooling mode.